Talk to your child about what might happen if he lies about things. It can not only label them as a dishonest person but will also prevent others from trusting him in future. You can also put a warning system in place which will prevent him from taking the path of dishonesty. Avoid scolding and yelling, as it might make the matter worse.
If the matter gets serious, then you can also take professional help to inculcate honesty in your kid.